/* -*- coding: utf-8 -*-
 *
 * 366.cc: No.366 ロボットソート - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 1000;
const int MAX_K = 1000;

/* typedef */

typedef vector<int> vi;
typedef map<int,int> mii;

/* global variables */

int as[MAX_N], bs[MAX_N];
vi aks[MAX_K];

/* subroutines */

int merge_sort(vi &as, vi &tmp, int l, int r) { // [l, r)
  int ic = 0;

  if (l + 1 < r) {
    int m = (l + r) / 2;
  
    ic = merge_sort(as, tmp, l, m) + merge_sort(as, tmp, m, r);

    int i = l, j = m, k = l;

    while (i < m && j < r) {
      if (as[i] <= as[j]) tmp[k++] = as[i++];
      else tmp[k++] = as[j++], ic += m - i;
    }

    while (i < m) tmp[k++] = as[i++];
    while (j < r) tmp[k++] = as[j++];

    for (int i = l; i < r; i++) as[i] = tmp[i];
  }  

  return ic;
}

/* main */

int main() {
  int n, k;
  cin >> n >> k;

  for (int i = 0; i < n; i++) (cin >> as[i]), bs[i] = as[i];
  sort(bs, bs + n);

  mii amap;
  for (int i = 0; i < n; i++) amap[bs[i]] = i;
  for (int i = 0; i < n; i++) {
    if (amap[as[i]] % k != i % k) {
      puts("-1");
      return 0;
    }

    aks[i % k].push_back(as[i]);
  }

  int sum = 0;
  for (int i = 0; i < k; i++) {
    int an = aks[i].size();
    vi tmp(an);
    sum += merge_sort(aks[i], tmp, 0, an);
  }

  printf("%d\n", sum);
  return 0;
}